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. Author manuscript; available in PMC: 2017 Jan 1.
Published in final edited form as: Transl Res. 2015 Jun 23;167(1):7–34. doi: 10.1016/j.trsl.2015.06.011

Figure 1.

Figure 1

The heme oxygenase (HO) reaction cleaves heme at the α-methene bridge carbon and generates carbon monoxide (CO), biliverdin-IXα. and ferrous iron (Fe II). The reaction proceeds through three sequential oxidation steps each requiring one mole of molecular oxygen (O2), and a total of seven electrons from NADPH: cytochrome p450 reductase. Three reaction intermediates have been proposed: α-meso-hydroxyheme, verdoheme, and the Fe (III)-biliverdin complex. Upon univalent reduction, the Fe (III)-biliverdin complex dissociates to form biliverdin-IXα and free Fe (II). The completion of enzymatic heme degradation involves the divalent reduction of biliverdin-IXα by NAD(P)H: biliverdin reductase (BVR; E.C. 1.3.1.24), which produces the lipid soluble pigment bilirubin-IXα. Heme side chains are designated: M=Methyl, V=Vinyl, P=Propionate.